The main goal of this project is the understanding of host defense mechanisms involved in recovery from virus-induced leukemia in mice. We have used selected breeding of mouse strains to isolate and characterize individual genetically controlled defense mechanisms and to study their actions and interactions during recovery from virus infection and/or virus-induced leukemia. In addition, we have analyzed a group of leukemia cell variants with alterations in viral expression. These are being characterized biochemically and immunologically to determine the roles of expression of different viral proteins in immune recognition and the possible relevance of this variation in expression to persistence of virus infected cells in vivo.